In industry and commerce, it is frequently necessary to deal with liquids which must be kept under a gas or vapor pressure greater than atmospheric pressure. Filling of tanks or containers under these conditions requires specialized techniques. In particular, it is usually necessary to ensure that the tank or container does not become completely filled with liquid. A specified gas or vapor space must remain at the end of the filling process. The techniques currently known to those skilled in the art for ensuring that the required gas or vapor space remains in the tank have serious deficiencies, for example in such applications as filling of fuel tanks of motor vehicles which utilize propane or liquefied petroleum gas (LPG) as fuel.